Tonic GABAAreceptor-mediated currents in human brain

Novel psychoplastogen DM506 reduces cue-induced heroin-seeking and inhibits tonic GABA currents in the Prelimbic Cortex

Opioid use disorder is a major public health crisis that is manifested by persistent drug-seeking behavior and high relapse frequency. Most of the available treatments rely on targeting opioid receptors using small molecules that do not provide sustained symptom alleviation. Psychoplastogens are a novel class of non-opioid drugs that produce rapid and sustained effects on neuronal plasticity, intended to produce therapeutic benefits. Ibogalogs are synthetic derivatives of iboga alkaloids that lack hallucinogenic or adverse side effects. In the current study, we examine the therapeutic potential of DM506, a novel ibogalog lacking any cardiotoxic or hallucinogenic effects, in cue-induced seeking behavior following heroin self-administration. At a single systemic dose of 40 mg/kg, DM506 significantly decreased cue-induced seeking in both male and female rats at abstinence day 1 (AD1) following heroin self-administration. Upon re-testing for cue-induced seeking at AD14, we found that males receiving DM506 continued to show decreased cue-induced seeking, an effect not observed in females. Since there is evidence of psychedelics influencing tonic GABA currents, and opioid and psychoplastogen-mediated neuroadaptations in the medial prefrontal cortex (PrL) underlying its functional effects, we performed patch-clamp recordings on PrL slices of drug-naïve rats with an acute application or chronic incubation with DM506. Tonic GABA current was decreased in slices incubated with DM506 for 2 h. qPCR analysis did not reveal any differences in the mRNA levels of GABAA receptor α and δ subunits at AD14 in heroin and saline self-administered animals that received vehicle or DM506 at AD1. Overall, our data indicate that DM506 attenuates cue-induced heroin seeking and inhibits tonic GABA current in the prelimbic cortex.

In-silico testing of new pharmacology for restoring inhibition and human cortical function in depression

Reduced inhibition by somatostatin-expressing interneurons is associated with depression. Administration of positive allosteric modulators of α5 subunit-containing GABAA receptor (α5-PAM) that selectively target this lost inhibition exhibit antidepressant and pro-cognitive effects in rodent models of chronic stress. However, the functional effects of α5-PAM on the human brain in vivo are unknown, and currently cannot be assessed experimentally. We modeled the effects of α5-PAM on tonic inhibition as measured in human neurons, and tested in silico α5-PAM effects on detailed models of human cortical microcircuits in health and depression. We found that α5-PAM effectively recovered impaired cortical processing as quantified by stimulus detection metrics, and also recovered the power spectral density profile of the microcircuit EEG signals. We performed an α5-PAM dose-response and identified simulated EEG biomarker candidates. Our results serve to de-risk and facilitate α5-PAM translation and provide biomarkers in non-invasive brain signals for monitoring target engagement and drug efficacy.

Age-dependent increased sag amplitude in human pyramidal neurons dampens baseline cortical activity

Aging involves various neurobiological changes, although their effect on brain function in humans remains poorly understood. The growing availability of human neuronal and circuit data provides opportunities for uncovering age-dependent changes of brain networks and for constraining models to predict consequences on brain activity. Here we found increased sag voltage amplitude in human middle temporal gyrus layer 5 pyramidal neurons from older subjects and captured this effect in biophysical models of younger and older pyramidal neurons. We used these models to simulate detailed layer 5 microcircuits and found lower baseline firing in older pyramidal neuron microcircuits, with minimal effect on response. We then validated the predicted reduced baseline firing using extracellular multielectrode recordings from human brain slices of different ages. Our results thus report changes in human pyramidal neuron input integration properties and provide fundamental insights into the neuronal mechanisms of altered cortical excitability and resting-state activity in human aging.

Reduced inhibition in depression impairs stimulus processing in human cortical microcircuits

Cortical processing depends on finely tuned excitatory and inhibitory connections in neuronal microcircuits. Reduced inhibition by somatostatin-expressing interneurons is a key component of altered inhibition associated with treatment-resistant major depressive disorder (depression), which is implicated in cognitive deficits and rumination, but the link remains to be better established mechanistically in humans. Here we test the effect of reduced somatostatin interneuron-mediated inhibition on cortical processing in human neuronal microcircuits using a data-driven computational approach. We integrate human cellular, circuit, and gene expression data to generate detailed models of human cortical microcircuits in health and depression. We simulate microcircuit baseline and response activity and find a reduced signal-to-noise ratio and increased false/failed detection of stimuli due to a higher baseline activity in depression. We thus apply models of human cortical microcircuits to demonstrate mechanistically how reduced inhibition impairs cortical processing in depression, providing quantitative links between altered inhibition and cognitive deficits.

Tonic GABAA Receptor-Mediated Currents of Human Cortical GABAergic Interneurons Vary Amongst Cell Types

Persistent anion conductances through GABAA receptors (GABAARs) are important modulators of neuronal excitability. However, it is currently unknown how the amplitudes of these currents vary among different cell types in the human neocortex, particularly among diverse GABAergic interneurons. We have recorded 101 interneurons in and near layer 1 from cortical tissue surgically resected from both male and female patients, visualized 84 of them and measured tonic GABAAR currents in 48 cells with an intracellular [Cl-] of 65 mm and in the presence of 5 μm GABA. We compare these tonic currents among five groups of interneurons divided by firing properties and four types of interneuron defined by axonal distributions; rosehip, neurogliaform, stalked-bouton, layer 2-3 innervating and a pool of other cells. Interestingly, the rosehip cell, a type of interneuron only described thus far in human tissue, and layer 2-3 innervating cells exhibit larger tonic currents than other layer 1 interneurons, such as neurogliaform and stalked-bouton cells; the latter two groups showing no difference. The positive allosteric modulators of GABAARs allopregnanolone and DS2 also induced larger current shifts in the rosehip and layer 2-3 innervating cells, consistent with higher expression of the δ subunit of the GABAAR in these neurons. We have also examined how patient parameters, such as age, seizures, type of cancer and anticonvulsant treatment may alter tonic inhibitory currents in human neurons. The cell type-specific differences in tonic inhibitory currents could potentially be used to selectively modulate cortical circuitry.SIGNIFICANCE STATEMENT Tonic currents through GABAA receptors (GABAARs) are a potential therapeutic target for a number of neurologic and psychiatric conditions. Here, we show that these currents in human cerebral cortical GABAergic neurons display cell type-specific differences in their amplitudes which implies differential modulation of their excitability. Additionally, we examine whether the amplitudes of the tonic currents measured in our study show any differences between patient populations, finding some evidence that age, seizures, type of cancer, and anticonvulsant treatment may alter tonic inhibition in human tissue. These results advance our understanding of how pathology affects neuronal excitability and could potentially be used to selectively modulate cortical circuitry.

Sleep Deprivation Exacerbates Seizures and Diminishes GABAergic Tonic Inhibition

Patients with epilepsy report that sleep deprivation is a common trigger for breakthrough seizures. The basic mechanism of this phenomenon is unknown. In the Kv1.1-/- mouse model of epilepsy, daily sleep deprivation indeed exacerbated seizures though these effects were lost after the third day. Sleep deprivation also accelerated mortality in ~ 52% of Kv1.1-/- mice, not observed in controls. Voltage-clamp experiments on the day after recovery from sleep deprivation showed reductions in GABAergic tonic inhibition in dentate granule cells in epileptic Kv1.1-/- mice. Our results suggest that sleep deprivation is detrimental to seizures and survival, possibly due to reductions in GABAergic tonic inhibition. ANN NEUROL 2021;90:840-844.

Increased expression of GABAA receptor subunits associated with tonic inhibition in patients with temporal lobe epilepsy

Epilepsy animal models indicate pronounced changes in the expression and rearrangement of GABA_A receptor subunits in the hippocampus and in para-hippocampal areas, including widespread downregulation of the subunits α5 and δ, and upregulation of α4, subunits that mediate tonic inhibition of GABA. In this case–control study, we investigated changes in the expression of subunits α4, α5 and δ in hippocampal specimens of drug resistant temporal lobe epilepsy patients who underwent epilepsy surgery. Using in situ hybridization, immunohistochemistry and α5-specific receptor autoradiography, we characterized expression of the receptor subunits in specimens from patients with and without Ammon’s horn sclerosis compared to post-mortem controls. Expression of the α5-subunit was abundant throughout all subfields of the hippocampus, including the dentate gyrus, sectors CA1 and CA3, the subiculum and pre- and parasubiculum. Significant but weaker expression was detected for subunits α4 and δ notably in the granule cell/molecular layer of control specimens, but was faint in the other parts of the hippocampus. Expression of all three subunits was similarly altered in sclerotic and non-sclerotic specimens. Respective mRNA levels were increased by about 50–80% in the granule cell layer compared with post-mortem controls. Subunit α5 mRNA levels and immunoreactivities were also increased in the sector CA3 and in the subiculum. Autoradiography for α5-containing receptors using [³H]L-655,708 as ligand showed significantly increased binding in the molecular layer of the dentate gyrus in non-sclerotic specimens. Increased expression of the α5 and δ subunits is in contrast to the previously observed downregulation of these subunits in different epilepsy models, whereas increased expression of α4 in temporal lobe epilepsy patients is consistent with that in the rodent models. Our findings indicate increased tonic inhibition likely representing an endogenous anticonvulsive mechanism in temporal lobe epilepsy.

Cenobamate, a Sodium Channel Inhibitor and Positive Allosteric Modulator of GABAA Ion Channels, for Partial Onset Seizures in Adults: A Comprehensive Review and Clinical Implications

Medical management of epilepsy seeks to eliminate or to reduce the frequency of seizures, help patients maintain a normal lifestyle, and maintain psychosocial and occupational activities, while avoiding the negative side effects of long-term treatment. Current FDA approved drugs have been shown to have similar efficacy; however, they all share a commonality of having side effects that have the potential to significantly reduce a patient’s quality of life. Cenobamate, a newly-FDA approved drug used to treat partial-onset seizures in adult patients, has demonstrated promise in that it works on two proposed mechanisms that are commonly associated with epilepsy. Cenobamate acts as a positive allosteric modulator of the GABA_A ion channels and is effective in reducing repetitive neuronal firing by inhibition of voltage-gated sodium channels, although the complete mechanism of action is currently unknown. The efficacy of Cenobamate with its low toxicity and adverse drug reaction profile emphasizes the need to further evaluate antiepileptic therapies containing sulfamoylphenyl and/or carbamate moieties in their chemical structure. Recent studies have found more patients to be seizure free during the maintenance period when compared to placebo. The most common side effects reported in with Cenobamate are somnolence, dizziness, headache, nausea, and fatigue. There are currently ongoing phase III studies looking to further evaluate the long-term benefits of Cenobamate and investigate adverse events.

Induction of Inhibitory Synaptic Plasticity Enhances Tonic Current by Increasing the Content of α5-Subunit Containing GABAA Receptors in Hippocampal Pyramidal Neurons

It is known that besides synaptic inhibition, there is a persistent component of inhibitory drive mediated by tonic currents which is believed to mediate majority of the total inhibitory charge in hippocampal neurons. Tonic currents, depending on cell types, can be mediated by a variety of GABA_A receptor (GABA_AR) subtypes but in pyramidal neurons, α5-subunit containing receptors were found to be predominant. Importantly, α5-GABA_ARs were implicated in both inhibitory and excitatory synaptic plasticity as well as in a variety of cognitive tasks. In the present study, we asked whether the protocol that evokes NMDAR-dependent GABAergic inhibitory long-term potentiation (iLTP) also induces the plasticity of tonic inhibition in hippocampal pyramidal neurons. Our whole-cell patch-clamp recordings revealed that the induction of this type of iLTP is associated with a marked increase in tonic current. By using the specific inverse agonist of α5-containing GABA_ARs (L-655,709) we provide evidence that this plastic change in tonic current is correlated with an increased proportion of this type of GABA_ARs. On the contrary, the iLTP induction did not affect the tonic current potentiated by THIP, indicating that the pool of δ subunit-containing GABA_ARs receptors remains unaffected. We conclude that the α5-GABA_ARs-dependent plasticity of tonic inhibition is a novel dimension of the neuroplasticity of the inhibitory drive in the hippocampal principal neurons. Overall, α5-containing GABA_ARs emerge as key players in a variety of plasticity mechanisms operating over a large span of time and spatial scales.

Astrocytic GABA Accumulation in Experimental Temporal Lobe Epilepsy

An imbalance of excitation and inhibition has been associated with the pathophysiology of epilepsy. Loss of GABAergic interneurons and/or synaptic inhibition has been shown in various epilepsy models and in human epilepsy. Despite this loss, several studies reported preserved or increased tonic GABA_A receptor-mediated currents in epilepsy, raising the question of the source of the inhibitory transmitter. We used the unilateral intracortical kainate mouse model of temporal lobe epilepsy (TLE) with hippocampal sclerosis (HS) to answer this question. In our model we observed profound loss of interneurons in the sclerotic hippocampal CA1 region and dentate gyrus already 5 days after epilepsy induction. Consistent with the literature, the absence of interneurons caused no reduction of tonic inhibition of CA1 pyramidal neurons. In dentate granule cells the inhibitory currents were even increased in epileptic tissue. Intriguingly, immunostaining of brain sections from epileptic mice with antibodies against GABA revealed strong and progressive accumulation of the neurotransmitter in reactive astrocytes. Pharmacological inhibition of the astrocytic GABA transporter GAT3 did not affect tonic inhibition in the sclerotic hippocampus, suggesting that this transporter is not responsible for astrocytic GABA accumulation or release. Immunostaining further indicated that both decarboxylation of glutamate and putrescine degradation accounted for the increased GABA levels in reactive astrocytes. Together, our data provide evidence that the preserved tonic inhibitory currents in the epileptic brain are mediated by GABA overproduction and release from astrocytes. A deeper understanding of the underlying mechanisms may lead to new strategies for antiepileptic drug therapy.

Isobolographic Analysis of Antiseizure Activity of the GABA Type A Receptor-Modulating Synthetic Neurosteroids Brexanolone and Ganaxolone with Tiagabine and Midazolam

Epilepsy is often treated with a combination of antiepileptic drugs. Although neurosteroids are potent anticonvulsants, little is known about their combination potential for the treatment of refractory epilepsy. Here, we investigated the combination efficacy of neurosteroids allopregnanolone (AP, brexanolone) and ganaxolone (GX) with the GABA-reuptake inhibitor tiagabine (TG) or the benzodiazepine midazolam (MDZ) on tonic inhibition in dentate gyrus granule cells and seizure protection in the hippocampus kindling and 6-Hz seizure models. Isobolographic analysis indicated that combinations of GX and TG or AP and TG at three standard ratios (1:1, 3:1, and 1:3) displayed significant synergism in augmenting tonic inhibition. In pharmacological studies, GX, AP, and TG produced dose-dependent antiseizure effects in mice (ED₅₀ = 1.46, 4.20, and 0.20 mg/kg, respectively). The combination of GX and TG at the fixed ratio of 1:1 exerted the greatest combination index (CI = 0.53), indicating strong synergistic interaction in seizure protection. In addition, combination regimens of AP and TG showed robust synergism for seizure protection (CI = 0.4). Finally, combination regimens of GX and MDZ elicited synergistic (CI = 0.6) responses for seizure protection. These results demonstrate striking synergism of neurosteroids and TG combination for seizure protection, likely because of their effects at extrasynaptic GABA type A (GABA-A) receptors from TG-induced elevation in GABA levels. Superadditive antiseizure activity of neurosteroid-MDZ combinations may stem from their actions at both synaptic and extrasynaptic GABA-A receptors. Together, these findings provide a potential mechanistic basis for combination potential of neurosteroids with TG or benzodiazepines for the management of refractory epilepsy, status epilepticus, and seizure disorders. SIGNIFICANCE STATEMENT: This paper investigates for the first time the potential synergistic interactions between two neurosteroids with anticonvulsant properties, allopregnanolone (brexanolone) and the very similar synthetic analog, ganaxolone, and two conventional antiepileptic drugs active at GABA type A receptors: the GABA-reuptake inhibitor tiagabine and a benzodiazepine, midazolam. The results demonstrate a synergistic protective effect of neurosteroid-tiagabine combinations, as well as neurosteroid-midazolam regimens in seizure models.

Bestrophin1-mediated tonic GABA release from reactive astrocytes prevents the development of seizure-prone network in kainate-injected hippocampi

Tonic extrasynaptic GABA_A receptor (GABA_A R) activation is under the tight control of tonic GABA release from astrocytes to maintain the brain's excitation/inhibition (E/I) balance; any slight E/I balance disturbance can cause serious pathological conditions including epileptic seizures. However, the pathophysiological role of tonic GABA release from astrocytes has not been tested in epileptic seizures. Here, we report that pharmacological or genetic intervention of the GABA-permeable Bestrophin-1 (Best1) channel prevented the generation of tonic GABA inhibition, disinhibiting CA1 pyramidal neuronal firing and augmenting seizure susceptibility in kainic acid (KA)-induced epileptic mice. Astrocyte-specific Best1 over-expression in KA-injected Best1 knockout mice fully restored the generation of tonic GABA inhibition and effectively suppressed seizure susceptibility. We demonstrate for the first time that tonic GABA from reactive astrocytes strongly contributes to the compensatory shift of E/I balance in epileptic hippocampi, serving as a good therapeutic target against altered E/I balance in epileptic seizures.

Reducing variability in motor cortex activity at a resting state by extracellular GABA for reliable perceptual decision-making

Interaction between sensory and motor cortices is crucial for perceptual decision-making, in which intracortical inhibition might have an important role. We simulated a neural network model consisting of a sensory network (N_S) and a motor network (N_M) to elucidate the significance of their interaction in perceptual decision-making in association with the level of GABA in extracellular space: extracellular GABA concentration. Extracellular GABA molecules acted on extrasynaptic receptors embedded in membranes of pyramidal cells and suppressed them. A reduction in extracellular GABA concentration either in N_S or N_M increased the rate of errors in perceptual decision-making, for which an increase in ongoing-spontaneous fluctuations in subthreshold neuronal activity in N_M prior to sensory stimulation was responsible. Feedback (N_M-to-N_S) signaling enhanced selective neuronal responses in N_S, which in turn increased stimulus-evoked neuronal activity in N_M. We suggest that GABA in extracellular space contributes to reducing variability in motor cortex activity at a resting state and thereby the motor cortex can respond correctly to a subsequent sensory stimulus. Feedback signaling from the motor cortex improves the selective responsiveness of the sensory cortex, which ensures the fidelity of information transmission to the motor cortex, leading to reliable perceptual decision-making.

Identification of transthyretin as a novel interacting partner for the δ subunit of GABAA receptors

GABA_A receptors (GABA_A-Rs) play critical roles in brain development and synchronization of neural network activity. While synaptic GABA_A-Rs can exert rapid inhibition, the extrasynaptic GABA_A-Rs can tonically inhibit neuronal activity due to constant activation by ambient GABA. The δ subunit-containing GABA_A-Rs are expressed abundantly in the cerebellum, hippocampus and thalamus to mediate the major tonic inhibition in the brain. While electrophysiological and pharmacological properties of the δ-GABA_A-Rs have been well characterized, the molecular interacting partners of the δ-GABA_A-Rs are not clearly defined. Here, using a yeast two-hybrid screening assay, we identified transthyretin (TTR) as a novel regulatory molecule for the δ-GABA_A-Rs. Knockdown of TTR in cultured cerebellar granule neurons significantly decreased the δ receptor expression; whereas overexpressing TTR in cortical neurons increased the δ receptor expression. Electrophysiological analysis confirmed that knockdown or overexpression of TTR in cultured neurons resulted in a corresponding decrease or increase of tonic currents. Furthermore, in vivo analysis of TTR-/- mice revealed a significant decrease of the surface expression of the δ-GABA_A-Rs in cerebellar granule neurons. Together, our studies identified TTR as a novel regulator of the δ-GABA_A-Rs.

Insulin Regulates GABAA Receptor-Mediated Tonic Currents in the Prefrontal Cortex

Recent studies, have shown that insulin increases extrasynaptic GABA_A receptor-mediated currents in the hippocampus, causing alterations of neuronal excitability. The prefrontal cortex (PFC) is another brain area which is involved in cognition functions and expresses insulin receptors. Here, we used electrophysiological, molecular, and immunocytochemical techniques to examine the effect of insulin on the extrasynaptic GABA_A receptor-mediated tonic currents in brain slices. We found that insulin (20–500 nM) increases GABA_A-mediated tonic currents. Our results suggest that insulin promotes the trafficking of extrasynaptic GABA_A receptors from the cytoplasm to the cell membrane. Western blot analysis and immunocytochemistry showed that PFC extrasynaptic GABA_A receptors contain α-5 and δ subunits. Insulin effect on tonic currents decreased the firing rate and neuronal excitability in layer 5–6 PFC cells. These effects of insulin were dependent on the activation of the PI3K enzyme, a key mediator of the insulin response within the brain. Taken together, these results suggest that insulin modulation of the GABA_A-mediated tonic currents can modify the activity of neural circuits within the PFC. These actions could help to explain the alterations of cognitive processes associated with changes in insulin signaling.

Commonalities in epileptogenic processes from different acute brain insults: Do they translate?

The most common forms of acquired epilepsies arise following acute brain insults such as traumatic brain injury, stroke, or central nervous system infections. Treatment is effective for only 60%-70% of patients and remains symptomatic despite decades of effort to develop epilepsy prevention therapies. Recent preclinical efforts are focused on likely primary drivers of epileptogenesis, namely inflammation, neuron loss, plasticity, and circuit reorganization. This review suggests a path to identify neuronal and molecular targets for clinical testing of specific hypotheses about epileptogenesis and its prevention or modification. Acquired human epilepsies with different etiologies share some features with animal models. We identify these commonalities and discuss their relevance to the development of successful epilepsy prevention or disease modification strategies. Risk factors for developing epilepsy that appear common to multiple acute injury etiologies include intracranial bleeding, disruption of the blood-brain barrier, more severe injury, and early seizures within 1 week of injury. In diverse human epilepsies and animal models, seizures appear to propagate within a limbic or thalamocortical/corticocortical network. Common histopathologic features of epilepsy of diverse and mostly focal origin are microglial activation and astrogliosis, heterotopic neurons in the white matter, loss of neurons, and the presence of inflammatory cellular infiltrates. Astrocytes exhibit smaller K+ conductances and lose gap junction coupling in many animal models as well as in sclerotic hippocampi from temporal lobe epilepsy patients. There is increasing evidence that epilepsy can be prevented or aborted in preclinical animal models of acquired epilepsy by interfering with processes that appear common to multiple acute injury etiologies, for example, in post-status epilepticus models of focal epilepsy by transient treatment with a trkB/PLCγ1 inhibitor, isoflurane, or HMGB1 antibodies and by topical administration of adenosine, in the cortical fluid percussion injury model by focal cooling, and in the albumin posttraumatic epilepsy model by losartan. Preclinical studies further highlight the roles of mTOR1 pathways, JAK-STAT3, IL-1R/TLR4 signaling, and other inflammatory pathways in the genesis or modulation of epilepsy after brain injury. The wealth of commonalities, diversity of molecular targets identified preclinically, and likely multidimensional nature of epileptogenesis argue for a combinatorial strategy in prevention therapy. Going forward, the identification of impending epilepsy biomarkers to allow better patient selection, together with better alignment with multisite preclinical trials in animal models, should guide the clinical testing of new hypotheses for epileptogenesis and its prevention.

Neuronal life after death: electrophysiologic recordings from neurons in adult human brain tissue obtained through surgical resection or postmortem

Recordings from fresh human brain slices derived from surgically resected brain tissue are being used to unravel mechanisms underlying human neurophysiology and for the evaluation of potential therapeutic targets and compounds. Data resulting from these studies provide unique insights into physiologic properties of human neuronal microcircuits. However, substantial limitations still remain with this approach. First, the tissue is always resected from patients, never from healthy controls. Second, the patient population undergoing brain surgery with tissue resection is limited to epilepsy and tumor patients - never from patients with other neurologic disorders. Third, the vast majority of tissue resected is limited largely to temporal cortex and hippocampus, occasionally amygdala. Therefore, the possibility to study brain tissue: (1) from healthy controls; (2) from patients with different neuropathologies; (3) from different brain areas; and (4) from a wide spectrum of ages only exists through autopsy-derived brain tissue. Here we describe methods and results from physiologic recordings of adult human neurons and microcircuits in both surgically derived brain tissue as well as in tissue derived from autopsies. We define postmortem time windows during which physiologic recordings could match data obtained from surgical tissue.

Improved Perceptual Learning by Control of Extracellular GABA Concentration by Astrocytic Gap Junctions

Learning of sensory cues is believed to rely on synchronous pre- and postsynaptic neuronal firing. Evidence is mounting that such synchronicity is not merely caused by properties of the underlying neuronal network but could also depend on the integrity of gap junctions that connect neurons and astrocytes in networks too. In this perspective, we set out to investigate the effect of astrocytic gap junctions on perceptual learning, introducing a model for coupled neuron-astrocyte networks. In particular, we focus on the fact that astrocytes are rich of GABA transporters (GATs) which can either uptake or release GABA depending on the astrocyte membrane potential, which is a function of local neural activity. We show that GABAergic signaling is a crucial component of intracolumnar neuronal synchronization, thereby promoting learning by neurons in the same cell assembly that are activated by a shared sensory cue. At the same time, we show that this effect can critically depend on astrocytic gap junctions insofar as these latter could synchronize extracellular GABA levels around many neurons and throughout entire cell assemblies. These results are supported by extensive computational arguments and predict that astrocytic gap junctions could improve perceptual learning by controlling extracellular GABA.

Extensive astrocyte synchronization advances neuronal coupling in slow wave activity in vivo

Slow wave activity (SWA) is a characteristic brain oscillation in sleep and quiet wakefulness. Although the cell types contributing to SWA genesis are not yet identified, the principal role of neurons in the emergence of this essential cognitive mechanism has not been questioned. To address the possibility of astrocytic involvement in SWA, we used a transgenic rat line expressing a calcium sensitive fluorescent protein in both astrocytes and interneurons and simultaneously imaged astrocytic and neuronal activity in vivo. Here we demonstrate, for the first time, that the astrocyte network display synchronized recurrent activity in vivo coupled to UP states measured by field recording and neuronal calcium imaging. Furthermore, we present evidence that extensive synchronization of the astrocytic network precedes the spatial build-up of neuronal synchronization. The earlier extensive recruitment of astrocytes in the synchronized activity is reinforced by the observation that neurons surrounded by active astrocytes are more likely to join SWA, suggesting causality. Further supporting this notion, we demonstrate that blockade of astrocytic gap junctional communication or inhibition of astrocytic Ca²⁺ transients reduces the ratio of both astrocytes and neurons involved in SWA. These in vivo findings conclusively suggest a causal role of the astrocytic syncytium in SWA generation.

Anticonvulsive evaluation of THIP in the murine pentylenetetrazole kindling model: lack of anticonvulsive effect of THIP despite functional δ-subunit-containing GABAA receptors in dentate gyrus granule cells

THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridin‐3‐ol) is a GABA_A receptor agonist with varying potencies and efficacies at γ‐subunit‐containing receptors. More importantly, THIP acts as a selective superagonist at δ‐subunit‐containing receptors (δ‐GABA_ARs) at clinically relevant concentrations. Evaluation of THIP as a potential anticonvulsant has given contradictory results in different animal models and for this reason, we reevaluated the anticonvulsive properties of THIP in the murine pentylenetetrazole (PTZ) kindling model. As loss of δ‐GABA_AR in the dentate gyrus has been associated with several animal models of epilepsy, we first investigated the presence of functional δ‐GABA_A receptors. Both immunohistochemistry and Western blot data demonstrated that δ‐GABA_AR expression is not only present in the dentate gyrus, but also the expression level was enhanced in the early phase after PTZ kindling. Whole‐cell patch‐clamp studies in acute hippocampal brain slices revealed that THIP was indeed able to induce a tonic inhibition in dentate gyrus granule cells. However, THIP induced a tonic current of similar magnitude in the PTZ‐kindled mice compared to saline‐treated animals despite the observed upregulation of δ‐GABA_ARs. Even in the demonstrated presence of functional δ‐GABA_ARs, THIP (0.5–4 mg/kg) showed no anticonvulsive effect in the PTZ kindling model using a comprehensive in vivo evaluation of the anticonvulsive properties.

Plasticity in Single Axon Glutamatergic Connection to GABAergic Interneurons Regulates Complex Events in the Human Neocortex

In the human neocortex, single excitatory pyramidal cells can elicit very large glutamatergic EPSPs (VLEs) in inhibitory GABAergic interneurons capable of triggering their firing with short (3–5 ms) delay. Similar strong excitatory connections between two individual neurons have not been found in nonhuman cortices, suggesting that these synapses are specific to human interneurons. The VLEs are crucial for generating neocortical complex events, observed as single pyramidal cell spike-evoked discharge of cell assemblies in the frontal and temporal cortices. However, long-term plasticity of the VLE connections and how the plasticity modulates neocortical complex events has not been studied. Using triple and dual whole-cell recordings from synaptically connected human neocortical layers 2–3 neurons, we show that VLEs in fast-spiking GABAergic interneurons exhibit robust activity-induced long-term depression (LTD). The LTD by single pyramidal cell 40 Hz spike bursts is specific to connections with VLEs, requires group I metabotropic glutamate receptors, and has a presynaptic mechanism. The LTD of VLE connections alters suprathreshold activation of interneurons in the complex events suppressing the discharge of fast-spiking GABAergic cells. The VLEs triggering the complex events may contribute to cognitive processes in the human neocortex, and their long-term plasticity can alter the discharging cortical cell assemblies by learning.

Many microscale features in the human neocortex—a part of the brain involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, and language—are closely similar to those reported in experimental animals commonly used in neuroscience, like mice. However, the human neocortical neurons also exhibit specializations only reported in our species. One such feature is the capacity of excitatory principal cells to elicit firing in local inhibitory interneurons with a single action potential via very strong excitatory synapses. It has been suggested that this feature has specifically evolved to enhance coordinated firing of neuronal ensembles in higher brain functions. However, it is unknown how these circuits are modified by learning. Therefore, we investigated how these very strong excitatory synapses are changed, and if their impact on the firing of local inhibitory neurons is altered by repetitive action potentials mimicking learning-related activity. By recording in human neocortical slices, we report that the strong excitatory synapses on interneurons exhibit robust activity-dependent long-term plasticity. The plasticity also regulates the discharge of local interneurons driven by these synapses. Although these specialized synapses have only been reported in the human neocortex, their plasticity mechanism is evolutionarily conserved. We suggest that the strong synapses with robust plasticity have evolved to enhance complex brain functions and learning.

Elevated basal glutamate and unchanged glutamine and GABA in refractory epilepsy: Microdialysis study of 79 patients at the yale epilepsy surgery program

OBJECTIVE

Aberrant glutamate and γ-aminobutyric acid (GABA) neurotransmission contribute to seizure generation and the epileptic state. However, whether levels of these neurochemicals are abnormal in epileptic patients is unknown. Here, we report on interictal levels of glutamate, glutamine, and GABA in epilepsy patients at seizure onset and nonepileptic sites, cortical lesions, and from patients with poorly localized neocortical epilepsies.

METHODS

Subjects (n = 79) were medically refractory epilepsy patients undergoing intracranial electroencephalogram evaluation. Microdialysis probes (n = 125) coupled to depth electrodes were implanted within suspected seizure onset sites and microdialysis samples were obtained during interictal periods. Glutamate, glutamine, and GABA were measured using high-performance liquid chromatography. Probe locations were subsequently classified by consensus of expert epileptologists.

RESULTS

Glutamate levels were elevated in epileptogenic (p = 0.03; n = 7), nonlocalized (p < 0.001), and lesional cortical sites (p < 0.001) when compared to nonepileptogenic cortex. Glutamate was also elevated in epileptogenic (p < 0.001) compared to nonepileptogenic hippocampus. There were no statistical differences in GABA or glutamine, although GABA levels showed high variability across patients and groups.

INTERPRETATION

Our findings indicate that chronically elevated extracellular glutamate is a common pathological feature among epilepsies with different etiology. Contrary to our predictions, GABA and glutamine levels were not decreased in any of the measured areas. Whereas variability in GABA levels may in part be attributed to the use of GABAergic antiepileptic drugs, the stability in glutamine across patient groups indicate that extracellular glutamine levels are under tighter metabolic regulation than previously thought. Ann Neurol 2016;80:35-45.

Optogenetic control of human neurons in organotypic brain cultures

Optogenetics is one of the most powerful tools in neuroscience, allowing for selective control of specific neuronal populations in the brain of experimental animals, including mammals. We report, for the first time, the application of optogenetic tools to human brain tissue providing a proof-of-concept for the use of optogenetics in neuromodulation of human cortical and hippocampal neurons as a possible tool to explore network mechanisms and develop future therapeutic strategies.

Reduction of Trial-to-Trial Perceptual Variability by Intracortical Tonic Inhibition

Variability is a prominent characteristic of cognitive brain function. For instance, different trials of presentation of the same stimulus yield higher variability in its perception: subjects sometimes fail in perceiving the same stimulus. Perceptual variability could be attributable to ongoing-spontaneous fluctuation in neuronal activity prior to sensory stimulation. Simulating a cortical neural network model, we investigated the underlying neuronal mechanism of perceptual variability in relation to variability in ongoing-spontaneous neuronal activity. In the network model, populations of principal cells (cell assemblies) encode information about sensory features. Each cell assembly is sensitive to one particular feature stimulus. Transporters on GABAergic interneurons regulate ambient GABA concentration in a neuronal activity-dependent manner. Ambient GABA molecules activate extrasynaptic GABAa receptors on principal cells and interneurons, and provide them with tonic inhibitory currents. We controlled the variability of ongoing-spontaneous neuronal activity by manipulating the basal level of ambient GABA and assessed the perceptual performance of the network: detection of a feature stimulus. In an erroneous response, stimulus-irrelevant but not stimulus-relevant principal cells were activated, generating trains of action potentials. Perceptual variability, reflected in error rate in detecting the same stimulus that was presented repeatedly to the network, was increased as the variability in ongoing-spontaneous membrane potential among cell assemblies increased. Frequent, transient membrane depolarization below firing threshold was the major cause of the increased neuronal variability, for which a decrease in basal ambient GABA concentration was responsible. We suggest that ambient GABA in the brain may have a role in reducing the variability in ongoing-spontaneous neuronal activity, leading to a decrease in perceptual variability and therefore to reliable sensory perception.

Tonic GABAA Receptors as Potential Target for the Treatment of Temporal Lobe Epilepsy

Tonic GABA_A receptors are a subpopulation of receptors that generate long-lasting inhibition and thereby control network excitability. In recent years, these receptors have been implicated in various neurological and psychiatric disorders, including Parkinson’s disease, schizophrenia, and epilepsy. Their distinct subunit composition and function, compared to phasic GABA_A receptors, opens the possibility to specifically modulate network properties. In this review, the role of tonic GABA_A receptors in epilepsy and as potential antiepileptic target will be discussed.

Reduction in focal ictal activity following transplantation of MGE interneurons requires expression of the GABAA receptor α4 subunit

Despite numerous advances, treatment-resistant seizures remain an important problem. Loss of neuronal inhibition is present in a variety of epilepsy models and is suggested as a mechanism for increased excitability, leading to the proposal that grafting inhibitory interneurons into seizure foci might relieve refractory seizures. Indeed, transplanted medial ganglionic eminence interneuron progenitors (MGE-IPs) mature into GABAergic interneurons that increase GABA release onto cortical pyramidal neurons, and this inhibition is associated with reduced seizure activity. An obvious conclusion is that inhibitory coupling between the new interneurons and pyramidal cells underlies this effect. We hypothesized that the primary mechanism for the seizure-limiting effects following MGE-IP transplantation is the tonic conductance that results from activation of extrasynaptic GABA_A receptors (GABA_A-Rs) expressed on cortical pyramidal cells. Using in vitro and in vivo recording techniques, we demonstrate that GABA_A-R α4 subunit deletion abolishes tonic currents (I_tonic) in cortical pyramidal cells and leads to a failure of MGE-IP transplantation to attenuate cortical seizure propagation. These observations should influence how the field proceeds with respect to the further development of therapeutic neuronal transplants (and possibly pharmacological treatments).

Facilitation of neuronal responses by intrinsic default mode network activity

Default mode network (DMN) shows intrinsic, high-level activity at rest. We tested a hypothesis proposed for its role in sensory information processing: Intrinsic DMN activity facilitates neural responses to sensory input. A neural network model, consisting of a sensory network (Nsen) and a DMN, was simulated. The Nsen contained cell assemblies. Each cell assembly comprised principal cells, GABAergic interneurons (Ia, Ib), and glial cells. We let the Nsen carry out a perceptual task: detection of sensory stimuli. During DMN activation, glial cells were hyperpolarized by Ia-to-glia circuitry, by which glial membrane transporters imported GABA molecules from the extracellular space and decreased ambient GABA concentration. Acting on extrasynaptic GABA receptors, the decrease in ambient GABA concentration reduced inhibitory current in a tonic manner. This depolarized principal cells below their firing threshold during the ongoing spontaneous time period and accelerated their reaction speed to a sensory stimulus. During the stimulus presentation period, the Nsen inhibited the DMN and caused DMN deactivation. The DMN deactivation made Nsen Ia cells cease firing, thereby stopping the glial membrane hyperpolarization, quitting the GABA import, returning to the basal ambient GABA level, and thus enhancing global inhibition. Notably, the stimulus-relevant P cell firing could be maintained when GABAergic gliotransmission via Ia-glia signaling worked, decreasing ambient GABA concentration around the stimulus-relevant P cells. This enabled the Nsen to reliably detect the stimulus. We suggest that intrinsic default model network activity may accelerate the reaction speed of the sensory network by modulating its ongoing-spontaneous activity in a subthreshold manner. Ambient GABA contributes to achieve an optimal ongoing spontaneous subthreshold neuronal state, in which GABAergic gliotransmission triggered by the intrinsic default model network activity may play an important role.

Tonically balancing intracortical excitation and inhibition by GABAergic gliotransmission

For sensory cortices to respond reliably to feature stimuli, the balancing of neuronal excitation and inhibition is crucial. A typical example might be the balancing of phasic excitation within cell assemblies and phasic inhibition between cell assemblies. The former controls the gain of and the latter the tuning of neuronal responses. A change in ambient GABA concentration might affect the dynamic behavior of neurons in a tonic manner. For instance, an increase in ambient GABA concentration enhances the activation of extrasynaptic receptors, augments an inhibitory current, and thus inhibits neurons. When a decrease in ambient GABA concentration occurs, the tonic inhibitory current is reduced, and thus the neurons are relatively excited. We simulated a neural network model in order to examine whether and how such a tonic excitatory-inhibitory mechanism could work for sensory information processing. The network consists of cell assemblies. Each cell assembly, comprising principal cells (P), GABAergic interneurons (Ia, Ib), and glial cells (glia), responds to one particular feature stimulus. GABA transporters, embedded in glial plasma membranes, regulate ambient GABA levels. Hypothetical neuron-glia signaling via inhibitory (Ia-to-glia) and excitatory (P-to-glia) synaptic contacts was assumed. The former let transporters import (remove) GABA from the extracellular space and excited stimulus-relevant P cells. The latter let them export GABA into the extracellular space and inhibited stimulus-irrelevant P cells. The main finding was that the glial membrane transporter gave a combinatorial excitatory-inhibitory effect on P cells in a tonic manner, thereby improving the gain and tuning of neuronal responses. Interestingly, it worked cooperatively with the conventional, phasic excitatory-inhibitory mechanism. We suggest that the GABAergic gliotransmission mechanism may provide balanced intracortical excitation and inhibition so that the best perceptual performance of the cortex can be achieved.

Pathophysiological power of improper tonic GABA(A) conductances in mature and immature models

High-affinity extrasynaptic gamma-aminobutyric acid A (GABA_A) receptors are tonically activated by low and consistent levels of ambient GABA, mediating chronic inhibition against neuronal excitability (tonic inhibition) and the modulation of neural development. Synaptic (phasic) inhibition is spatially and temporally precise compared with tonic inhibition, which provides blunt yet strong integral inhibitory force by shunting electrical signaling. Although effects of acute modification of tonic inhibition are known, its pathophysiological significance remains unclear because homeostatic regulation of neuronal excitability can compensate for long-term deficit of extrasynaptic GABA_A receptor activation. Nevertheless, tonic inhibition is of great interest for its pathophysiological involvement in central nervous system (CNS) diseases and thus as a therapeutic target. Together with the development of experimental models for various pathological states, recent evidence demonstrates such pathological involvements of tonic inhibition in neuronal dysfunction. This review focuses on the recent progress of tonic activation of GABA_A conductance on the development and pathology of the CNS. Findings indicate that neuronal function in various brain regions are exacerbated with a gain or loss of function of tonic inhibition by GABA spillover. Disturbance of tonic GABA_A conductance mediated by non-synaptic ambient GABA may result in brain mal-development. Therefore, various pathological states (epilepsy, motor dysfunctions, psychiatric disorders, and neurodevelopmental disorders) may be partly attributable to abnormal tonic GABA_A conductances. Thus, the tone of tonic conductance and level of ambient GABA may be precisely tuned to maintain the regular function and development of the CNS. Therefore, receptor expression and factors for regulating the ambient GABA concentration are highlighted to gain a deeper understanding of pathology and therapeutic strategy for CNS diseases.

Ambient GABA Responsible for Age-Related Changes in Multistable Perception

Multistable perception is a psychophysical phenomenon in which one unique interpretation alternates spontaneously every few seconds between two or more interpretations of the same sensory input. Well-known examples include the Necker cube and face-vase illusions in vision. Interestingly, young adults generally see more perceptual switches than do elderly people. To understand the underlying neuronal mechanism of age-related multistable perception, we simulated a cortical neural network model that consists of multiple cell assemblies. In the network, a specific population of noncore cells and a common population of core cells form a cell assembly that represents a single object (or event). Every dynamic cell assembly, activated by a given sensory input, involves the common (overlapping) population of core cells. Ambient GABA-mediated intracortical tonic inhibition via extrasynaptic GABAa receptors destabilized the currently appearing dynamic cell assembly and terminated its burst firing. This allowed another dynamic cell assembly to emerge one after the other. Namely, multistable perception took place. Transporters, which were embedded in axon terminal membranes of interneurons, regulated levels of ambient GABA. For elderly people, we assumed a decline in transporter. This decelerated GABA augmentation and resulted in prolonging the durations of burst firing and thus in slowing perceptual switches. We suggest that poor control of ambient GABA levels due to age-related decline in GABA transporter may be responsible for the slowing of perceptual switches in elderly people.

Cortical inhibition, pH and cell excitability in epilepsy: what are optimal targets for antiepileptic interventions?

Epilepsy is characterised by the propensity of the brain to generate spontaneous recurrent bursts of excessive neuronal activity, seizures. GABA-mediated inhibition is critical for restraining neuronal excitation in the brain, and therefore potentiation of GABAergic neurotransmission is commonly used to prevent seizures. However, data obtained in animal models of epilepsy and from human epileptic tissue suggest that GABA-mediated signalling contributes to interictal and ictal activity. Prolonged activation of GABA(A) receptors during epileptiform bursts may even initiate a shift in GABAergic neurotransmission from inhibitory to excitatory and so have a proconvulsant action. Direct targeting of the membrane mechanisms that reduce spiking in glutamatergic neurons may better control neuronal excitability in epileptic tissue. Manipulation of brain pH may be a promising approach and recent advances in gene therapy and optogenetics seem likely to provide further routes to effective therapeutic intervention.

Regulation of Ambient GABA Levels by Neuron-Glia Signaling for Reliable Perception of Multisensory Events

Activities of sensory-specific cortices are known to be suppressed when presented with a different sensory modality stimulus. This is referred to as cross-modal inhibition, for which the conventional synaptic mechanism is unlikely to work. Interestingly, the cross-modal inhibition could be eliminated when presented with multisensory stimuli arising from the same event. To elucidate the underlying neuronal mechanism of cross-modal inhibition and understand its significance for multisensory information processing, we simulated a neural network model. Principal cell to and GABAergic interneuron to glial cell projections were assumed between and within lower-order unimodal networks (X and Y), respectively. Cross-modality stimulation of Y network activated its principal cells, which then depolarized glial cells of X network. This let transporters on the glial cells export GABA molecules into the extracellular space and increased a level of ambient (extrasynaptic) GABA. The ambient GABA molecules were accepted by extrasynaptic GABAa receptors and tonically inhibited principal cells of the X network. Cross-modal inhibition took place in a nonsynaptic manner. Identical modality stimulation of X network activated its principal cells, which then activated interneurons and hyperpolarized glial cells of the X network. This let their transporters import (remove) GABA molecules from the extracellular space and reduced tonic inhibitory current in principal cells, thereby improving their gain function. Top-down signals from a higher-order multimodal network (M) contributed to elimination of the cross-modal inhibition when presented with multisensory stimuli that arose from the same event. Tuning into the multisensory event deteriorated if the cross-modal inhibitory mechanism did not work. We suggest that neuron-glia signaling may regulate local ambient GABA levels in order to coordinate cross-modal inhibition and improve neuronal gain function, thereby achieving reliable perception of multisensory events.

Dynamic modulation of an orientation preference map by GABA responsible for age-related cognitive performance

Accumulating evidence suggests that cognitive declines in old (healthy) animals could arise from depression of intracortical inhibition, for which a decreased ability to produce GABA during senescence might be responsible. By simulating a neural network model of a primary visual cortical (V1) area, we investigated whether and how a lack of GABA affects cognitive performance of the network: detection of the orientation of a visual bar-stimulus. The network was composed of pyramidal (P) cells and GABAergic interneurons such as small (S) and large (L) basket cells. Intrasynaptic GABA-release from presynaptic S or L cells contributed to reducing ongoing-spontaneous (background) neuronal activity in a different manner. Namely, the former exerted feedback (S-to-P) inhibition and reduced the frequency (firing rate) of action potentials evoked in P cells. The latter reduced the number of saliently firing P cells through lateral (L-to-P) inhibition. Non-vesicular GABA-release, presumably from glia and/or neurons, into the extracellular space reduced the both, activating extrasynaptic GABAa receptors and providing P cells with tonic inhibitory currents. By this combinatorial, spatiotemporal inhibitory mechanism, the background activity as noise was significantly reduced, compared to the stimulus-evoked activity as signal, thereby improving signal-to-noise (S/N) ratio. Interestingly, GABA-spillover from the intrasynaptic cleft into the extracellular space was effective for improving orientation selectivity (orientation bias), especially when distractors interfered with detecting the bar-stimulus. These simulation results may provide some insight into how the depression of intracortical inhibition due to a reduction in GABA content in the brain leads to age-related cognitive decline.

The effects of volatile anesthetics on synaptic and extrasynaptic GABA-induced neurotransmission

Examination of volatile anesthetic actions at single synapses provides more direct information by reducing interference by surrounding tissue and extrasynaptic modulation. We examined how volatile anesthetics modulate GABA release by measuring spontaneous or miniature GABA-induced inhibitory postsynaptic currents (mIPSCs, sIPSCs) or by measuring action potential-evoked IPSCs (eIPSCs) at individual synapses. Halothane increased both the amplitude and frequency of sIPSCs. Isoflurane and enflurane increased mIPSC frequency while sevoflurane had no effect. These anesthetics did not alter mIPSC amplitudes. Halothane increased the amplitude of eIPSCs, with a decrease in failure rate (Rf) and paired-pulse ratio. In contrast, isoflurane and enflurane decreased the eIPSC amplitude and increased Rf, while sevoflurane decreased the eIPSC amplitude without affecting Rf. Volatile anesthetics did not change kinetics except for sevoflurane, suggesting that presynaptic mechanisms dominate changes in neurotransmission. Each anesthetic showed somewhat different GABA-induced response and these results suggest that GABA-induced synaptic transmission cannot have a uniformly common site of action as suggested for volatile anesthetics. In contrast, all volatile anesthetics concentration-dependently enhanced the GABA-induced extrasynaptic currents. Extrasynaptic receptors containing α4 and α5 subunits are reported to have high sensitivities to volatile anesthetics. Also, inhibition of GABA uptake by volatile anesthetics results in higher extracellular GABA concentration, which may lead to prolonged activation of extrasynaptic GABAA receptors. The extrasynaptic GABA-induced receptors may be major site of volatile anesthetic-induced neurotransmission. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Tonic neuromodulation of the inspiratory rhythm generator

The generation of neural network dynamics relies on the interactions between the intrinsic and synaptic properties of their neural components. Moreover, neuromodulators allow networks to change these properties and adjust their activity to specific challenges. Endogenous continuous (“tonic”) neuromodulation can regulate and sometimes be indispensible for networks to produce basal activity. This seems to be the case for the inspiratory rhythm generator located in the pre-Bötzinger complex (preBötC). This neural network is necessary and sufficient for generating inspiratory rhythms. The preBötC produces normal respiratory activity (eupnea) as well as sighs under normoxic conditions, and it generates gasping under hypoxic conditions after a reconfiguration process. The reconfiguration leading to gasping generation involves changes of synaptic and intrinsic properties that can be mediated by several neuromodulators. Over the past years, it has been shown that endogenous continuous neuromodulation of the preBötC may involve the continuous action of amines and peptides on extrasynaptic receptors. I will summarize the findings supporting the role of endogenous continuous neuromodulation in the generation and regulation of different inspiratory rhythms, exploring the possibility that these neuromodulatory actions involve extrasynaptic receptors along with evidence of glial modulation of preBötC activity.

Tonic GABA(A) receptor-mediated signalling in temporal lobe epilepsy

The tonic activation of extrasynaptic GABAA receptors by extracellular GABA provides a powerful means of regulating neuronal excitability. A consistent finding from studies that have used various models of temporal lobe epilepsy is that tonic GABAA receptor-mediated conductances are largely preserved in epileptic brain (in contrast to synaptic inhibition which is often reduced). Tonic inhibition is therefore an attractive target for antiepileptic drugs. However, the network consequences of a commonly used approach to augment tonic GABAA receptor-mediated conductances by global manipulation of extracellular GABA are difficult to predict without understanding how epileptogenesis alters the pharmacology and GABA sensitivity of tonic inhibition, and how manipulation of tonic conductances modulates the output of individual neurons. Here we review the current literature on epilepsy-associated changes in tonic GABAA receptor-mediated signalling, and speculate about possible effects they have at the network level. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Subthreshold Membrane Depolarization as Memory Trace for Perceptual Learning

Experience-dependent synaptic plasticity characterizes the adaptable brain and is believed to be the cellular substrate for perceptual learning. A chemical agent such as gamma-aminobutyric acid (GABA) is known to affect synaptic alteration, perhaps gating perceptual learning. We examined whether and how ambient (extrasynaptic) GABA affects experience-dependent synaptic alteration. A cortical neural network model was simulated. Transporters on GABAergic interneurons regulate ambient GABA levels around their axonal target neurons by removing GABA from (forward transport) or releasing it into (reverse transport) the extracellular space. The ambient GABA provides neurons with tonic inhibitory currents by activating extrasynaptic GABAa receptors. During repeated exposures to the same stimulus, we modified the synaptic connection strength between principal cells in a spike-timing-dependent manner. This modulated the activity of GABAergic interneurons, and reduced or augmented ambient GABA concentration. Reduction in ambient GABA concentration led to slight depolarization (less than several millivolts) in ongoing-spontaneous membrane potential. This was a subthreshold neuronal behavior because ongoing-spontaneous spiking activity remained almost unchanged. The ongoing-spontaneous subthreshold depolarization improved a suprathreshold neuronal response. If the stimulus was long absent for perceptual learning, augmentation of ambient GABA concentration took place and the ongoing-spontaneous subthreshold depolarization was depressed. We suggest that a perceptual memory trace could be left in neuronal circuitry as an ongoing-spontaneous subthreshold membrane depolarization, which would allow that memory to be accessed easily afterward, whereas a trace of a memory that has not recently been retrieved fades away when the ongoing-spontaneous subthreshold membrane depolarization built by previous perceptual learning is depressed. This would lead that memory to be accessed with some difficulty. In the brain, ambient GABA, whose level could be regulated by transporter may have an important role in leaving memory trace for perceptual learning.

Insufficient augmentation of ambient GABA responsible for age-related cognitive deficit

Age-related degeneration of intracortical inhibition could underlie declines in cognitive function during senescence. Based on a hypothesis that a decrease in basal concentration of ambient (extrasynaptic) GABA with aging leads to depressing intracortical inhibition, we investigated how the basal concentration affects stimulus-evoked activity (as signal), ongoing-spontaneous activity (as noise) of neurons and their (signal-to-noise) ratio S/N. We simulated a neural network model equipped with a GABA transport system that regulates ambient GABA concentration in a neuronal activity-dependent manner. An increase in basal concentration augmented ambient GABA, increased GABA-mediated inhibitory current, and depressed ongoing-spontaneous activity while still keeping stimulus-evoked activity. This led to S/N improvement, for which it was necessary for the reversal potential of GABA transporter to be close to the resting potential of neurons. Above the resting potential, ongoing-spontaneous activity was predominantly enhanced due to excessive GABA-uptake from the extracellular space by transporters. Below the resting potential, stimulus-evoked activity was predominantly depressed, caused by excessive GABA-release. We suggest that the insufficient augmentation of ambient GABA due to a decrease in its basal concentration may be one of the possible causes of cognitive deficit with aging, increasing ongoing-spontaneous neuronal activity as noise. GABA transporter may contribute to improving S/N, provided that its reversal potential is close to the resting potential.

Alteration of Ambient GABA by Phasic and Tonic Neuronal Activation

Neurons of primary auditory cortex (AI) emit spikes (action potentials) in two distinct manners, responding to sounds in an onset or a sustained manner. The former AI neurons are called phasic cells and the latter tonic cells. The phasic cells generate spikes for a brief time period (less than hundreds of milliseconds) at the onset of an auditory stimulus (e.g., a tone frequency sound), and the tonic cells continuously generate spikes throughout the stimulation period. Simulating a neural network model of AI, we investigated whether and how the onset discharges influence the sustained discharges that are believed to play a central role in encoding auditory information. Onset discharges, triggered by a phasic input, briefly excited GABAergic interneurons and transiently increased the level of ambient GABA, which was immediately recognized by extrasynaptic GABAa receptors and provided inhibitory currents into neurons. The transient alteration of ambient GABA allowed tonic cells to respond selectively to a tonic input. The timing of phasic input relative to a tonic one had a great impact on the responsiveness of tonic cells. We found optimal timing for the best selective responsiveness: phasic input preceding tonic input by several tens of milliseconds. Offset discharges induced by a secondary input to phasic cells, applied at the end of the tonic input period, suddenly terminated the sustained discharges and allowed the network to return rapidly to the ongoing-spontaneous neuronal state. We suggest that the transporter-mediated alteration of ambient GABA, triggered by onset discharges, may improve the response property of AI neurons. Offset discharges may have a role in resetting AI neurons so that they can prepare for the next auditory input.

Puberty, steroids and GABA(A) receptor plasticity

GABA(A) receptors (GABAR) mediate most inhibition in the CNS and are also a target for neuroactive steroids such as 3alpha,5[alpha]beta-THP (3alphaOH-5[alpha]beta-OH-pregnan-20-one or [allo]pregnanolone). Although these steroids robustly enhance current gated by alpha1beta2delta GABAR, we have shown that 3alpha,5[alpha]beta-THP effects at recombinant alpha4beta2delta GABAR depend on the direction of Cl(-) flux, where the steroid increases outward flux, but decreases inward flux through the receptor. This polarity-dependent inhibition of alpha4beta2delta GABAR resulted from an increase in the rate and extent of rapid desensitization of the receptor, recorded from recombinant receptors expressed in HEK-293 cells with whole cell voltage clamp techniques. This inhibitory effect of 3alpha,5[alpha]beta-THP was not observed at other receptor subtypes, suggesting it was selective for alpha4beta2delta GABAR. Furthermore, it was prevented by a selective mutation of basic residue arginine 353 in the intracellular loop of the receptor, suggesting that this might be a putative chloride modulatory site. Expression of alpha4betadelta GABAR increases markedly at extrasynaptic sites at the onset of puberty in female mice. At this time, 3alpha,5[alpha]beta-THP decreased the inhibitory tonic current, recorded with perforated patch techniques to maintain the physiological Cl(-) gradient. By decreasing this shunting inhibition, 3alpha,5[alpha]beta-THP increased the excitability of CA1 hippocampal pyramidal cells at puberty. These effects of the steroid were opposite to those observed before puberty when 3alpha,5[alpha]beta-THP reduced neuronal excitability as a pre-synaptic effect. Behaviorally, the excitatory effect of 3alpha,5[alpha]beta-THP was reflected as an increase in anxiety at the onset of puberty in female mice. Taken together, these findings suggest that the emergence of alpha4beta2delta GABAR at the onset of puberty reverses the effect of a stress steroid. These findings may be relevant for the mood swings and increased response to stressful events reported in adolescence.

Extrasynaptic GABAA receptors: form, pharmacology, and function

GABA is the principal inhibitory neurotransmitter in the CNS and acts via GABA(A) and GABA(B) receptors. Recently, a novel form of GABA(A) receptor-mediated inhibition, termed "tonic" inhibition, has been described. Whereas synaptic GABA(A) receptors underlie classical "phasic" GABA(A) receptor-mediated inhibition (inhibitory postsynaptic currents), tonic GABA(A) receptor-mediated inhibition results from the activation of extrasynaptic receptors by low concentrations of ambient GABA. Extrasynaptic GABA(A) receptors are composed of receptor subunits that convey biophysical properties ideally suited to the generation of persistent inhibition and are pharmacologically and functionally distinct from their synaptic counterparts. This mini-symposium review highlights ongoing work examining the properties of recombinant and native extrasynaptic GABA(A) receptors and their preferential targeting by endogenous and clinically relevant agents. In addition, it emphasizes the important role of extrasynaptic GABA(A) receptors in GABAergic inhibition throughout the CNS and identifies them as a major player in both physiological and pathophysiological processes.

GABA transporter preserving ongoing spontaneous neuronal activity at firing subthreshold

There has been compelling evidence that the GABA transporter is crucial not only for removing gamma-aminobutyric acid (GABA) from but also releasing it into extracellular space, thereby clamping ambient GABA (GABA in extracellular space) at a certain level. The ambient GABA is known to activate extrasynaptic GABA receptors and provide tonic inhibitory current into neurons. We investigated how the transporter regulates the level of ambient GABA, mediates tonic neuronal inhibition, and influences ongoing spontaneous neuronal activity. A cortical neural network model is proposed in which GABA transporters on lateral (L) and feedback (F) inhibitory (GABAergic) interneurons are functionally made. Principal (P) cell assemblies participate in expressing information about elemental sensory features. At membrane potentials below the reversal potential, there is net influx of GABA, whereas at membrane potentials above the reversal potential, there is net efflux of GABA. Through this transport mechanism, ambient GABA concentration is kept within a submicromolar range during an ongoing spontaneous neuronal activity time period. Here we show that the GABA transporter on L cells regulates the overall level of ambient GABA across cell assemblies, and that on F cells it does so within individual cell assemblies. This combinatorial regulation of ambient GABA allows P cells to oscillate near firing threshold during the ongoing time period, thereby reducing their reaction time to externally applied stimuli. We suggest that the GABA transporter, with its forward and reverse transport mechanism, could regulate the ambient GABA. This transporter-mediated ambient GABA regulation may contribute to establishing an ongoing subthreshold neuronal state by which the network can respond rapidly to subsequent sensory input.

Unanticipated structural and functional properties of delta-subunit-containing GABAA receptors

GABA(A) receptors mediate inhibitory neurotransmission in the mammalian brain via synaptic and extrasynaptic receptors. The delta (delta)-subunit-containing receptors are expressed exclusively extra-synaptically and mediate tonic inhibition. In the present study, we were interested in determining the architecture of receptors containing the delta-subunit. To investigate this, we predefined the subunit arrangement by concatenation. We prepared five dual and three triple concatenated subunit constructs. These concatenated dual and triple constructs were used to predefine nine different GABA(A) receptor pentamers. These pentamers composed of alpha(1)-, beta(3)-, and delta-subunits were expressed in Xenopus oocytes and maximal currents elicited in response to 1 mm GABA were determined in the presence and absence of THDOC (3alpha, 21-dihydroxy-5alpha-pregnane-20-one). beta(3)-alpha(1)-delta/alpha(1)-beta(3) and beta(3)-alpha(1)-delta/beta(3)-alpha(1) resulted in the expression of large currents in response to GABA. Interestingly, the presence of the neurosteroid THDOC uncovered alpha(1)-beta(3)-alpha(1)/beta(3)-delta receptors, additionally. The functional receptors were characterized in detail using the agonist GABA, THDOC, Zn(2+), and ethanol and their properties were compared with those of non-concatenated alpha(1)beta(3) and alpha(1)beta(3)delta receptors. Each concatenated receptor isoform displayed a specific set of properties, but none of them responded to 30 mm ethanol. We conclude from the investigated receptors that delta can assume multiple positions in the receptor pentamer. The GABA dose-response properties of alpha(1)-beta(3)-alpha(1)/beta(3)-delta and beta(3)-alpha(1)-delta/alpha(1)-beta(3) match most closely the properties of non-concatenated alpha(1)beta(3)delta receptors. Furthermore, we show that the delta-subunit can contribute to the formation of an agonist site in alpha(1)-beta(3)-alpha(1)/beta(3)-delta receptors.